                (1) Field of the Invention        
This invention relates to high speed laser scribing method of the fragile material such as fine glass used in flat panel display, quartz, ceramics or semiconductor. In order to simplify the explanation, the case of processing glass plate is represented below.
(2) Description of the Related Art
The cutting of fine glass plate used in flat panel displays which are in liquid crystal (hereafter abbreviated as LC) TV or plasma TV sets is presently performed using the conventional mechanical method, which is not free from various kinds of problems, such as the necessity of polishing, existence of a micro-crack layer, etc. The glass used in automobile, which is mostly round-shaped, requires polishing after the mechanical straight line cutting. Tempered glass used in architecture is difficult to cut mechanically and requires a new processing method.
Glass plate has been scribed until now using the mechanical method employing an ultra-hard tip such as a tip made of diamond. This method is accompanied by the following shortcomings. The first is the generation of cullet, which contaminates the glass surface. The second is the generation of micro-cracks in the processed area, which weakens the mechanical strength of glass. The third is the existence of kerf, which is as wide as a few hundreds μm in the smallest case and cannot be neglected in the processing of extremely small work chips. Other factors, such as the limit of processing speed and the cost of diamond tips, also cannot be neglected.
Different from the case of processing architectural glass plate, the scribing of fine glass plate such as that used in LC or plasma displays requires subsequent polishing and cleaning procedures for removing the micro crack zone.
On the other hand, the recently emerging laser scribing technology possesses the following advantages and is expected to eliminate the shortcomings possessed by the diamond tip method. The first is the cullet-free processing characteristics, so that a cleaning process is not required. The second is the absence of the generation of micro cracks, which results in high mechanical strength of the scribed area, so that subsequent polishing is not necessary. The third is the scribed surface, which is as perfect as a mirror-polished one. The fourth is the highly accurate shaping geometry, the error of which is smaller than ±25 μm. The fifth is ability to withstand the ever decreasing thickness of glass plate, which will find application in the future LC TV. Thus, the advent of the laser technology, which can improve the quality and broaden the range of processing, will be the solution to the various kinds of problem seen today.
Next, the principle of laser scribing is described. In the case of the irradiation of a very high power CO2 laser beam on the glass surface, strong absorption of the beam takes place at the spot of the irradiation. The rapid local heating invites random and irregular but mostly radially distributed cracks, and a controlled straight line scribing in the desired direction cannot be realized.
When the laser beam intensity is low enough only to heat the glass surface gently and not to change its property nor to melt it, then the glass, while struggling to expand but being pushed back by the surrounding glass, undergoes concentric compressional stress. The compressional stress takes the maximum value at the beam center and decreases as the distance from the beam center increases. The compressional stress is transformed radially from the beam center toward surroundings in the glass plate with almost the speed of sound. As is known, when the compressional stress exists in a plate, a tensile stress is generated with its direction being tangential proportional to the Poisson ratio, which is illustrated in FIG. 1. In FIG. 1, curves of a radially directed stress σx and a tangentially directed stress σy are illustrated. The radially directed stress σx is always a compression stress with negative value in FIG. 1 and the tangentially directed stress σy is a compression stress at the beam center and changes into a tensile one apart from the beam center.
Between the compression stress and the tensile stress, the tensile stress is influential with scribing the glass plate. When the tensile stress exceeds a cleavage toughness, which is a characteristic value of the glass, uncontrollable destructions or breaks occur everywhere in the glass plate. In the laser scribing, the maximum value of the tensile stress is selected as being below the cleavage toughness of the material so that the uncontrollable destructions or breaks do not occur.
When a crack is provided at a position where the tensile stress occurs, the stress at its tip is magnified. In this case, when this magnified value is selected to be greater than the cleavage toughness, the crack tip will be cleaved further. As a result, the crack extends from its tip towards the beam center in such a manner that controlled scribing is generated and the scribing proceeds thereby. When the laser beam is scanned over the glass plate, the scribing proceeds on the straight line connecting the crack tip and the laser beam center. This is the controlled scribing, which is called “thermal stress scribing” of fragile material. This scribing is usually accelerated by applying a cooling procedure. The laser scribing yields scribed surfaces similar to cleavage surfaces of a crystal, such that no micro-crack or cullet is present.
U.S. Pat. No. 5,609,284 has been known as a typical patent for the laser scribing method for the glass plate. The patent describes that the glass plate is heated by an incident laser beam of radiation to a temperature short of its softening point, with the rate of relative displacement of the beam and of the glass plate, and the region of the heated zone which is locally cooled being selected to form a blind crack in the glass plate.
FIG. 2 shows a principal figure for the laser scribing method of the U.S. Pat. No. 5,609,284. A CO2 laser beam is employed as the heating laser beam for the glass plate. The laser beam is scanned in a scanning direction 7. About 99% of the energy of the laser beam spot 1 of the CO2 laser beam is absorbed at surface region of a glass plate 6 and not transmitted throughout the glass plate 6. This is a result of the extremely large absorption coefficient of the glass for the CO2 laser light wavelength. As a result, heating of the glass plate occurs only at the surface region of the glass plate 6 and a compressional stress 4 is generated at the heated region of the glass plate. A point 3 apart from the heated region by the laser beam spot 1 is cooled by a suitable coolant. Then, a tensile stress 2 is generated, whereby a surface scribing 5 is generated behind the cooled point 3 which is extended from an initial crack 8.
The depth of the surface scribing 5 is usually no more than 100 μm if there exists an aid of thermal conducting towards depth direction in the glass plate 6. However, the glass plate 6 has such strong fragile characteristics that the glass plate 6 is breakable mechanically by applying bending stress in line with the surface scribing 5 of the glass plate. In order to separate the glass plate 6 completely, the application of a mechanical or bending stress on an un-scribed plane remaining underneath the surface scribing 5 is required. Here, the process for cutting the glass plate 6 completely by applying the mechanical or the bending stress in line with the surface scribing 5 is called “mechanical breaking”.
In contrast to the case of the mechanical scribing, in which very poor processing quality is obtained, the laser scribing offers ideally high quality results both in laser scribed and mechanically broken layers. There is a clear boundary, however, seen microscopically between both the layers.
FIG. 3 shows a figure for the laser scribing method of Japanese Patent No. 3792639. The patent employs five points linear arrayed laser beam 9 instead of the laser beam 1 shown in FIG. 2, and a distance G between heating area H by the five points linear arrayed laser beam 9 and cooling point 3 is variable. The distance G may be determined experimentally as an optimum value and thereby breaking characteristics may be further improved.
FIG. 4 shows a figure of a beam splitter which explains a method for generating five points linear arrayed laser beam 9. A laser beam B emitted from a laser oscillator (not shown in FIG. 4) is transmitted into the beam splitter 14 through a non-reflective surface 11 thereof. Then, the laser beam B is partially reflected by a partial reflective surface 13 of the beam splitter 14 and split into an outgoing laser beam b1 and a partially reflected laser beam returned into the beam splitter 14. The partially reflected laser beam is reflected by a total reflection surface 10 of the beam splitter 14 and partially reflected again by the partial reflective surface 13 of the beam splitter 14 so as to split into an outgoing laser beam b2 and a partially reflected laser beam returned into the beam splitter 14 in a similar way. By repeating this process four times, five outgoing laser beams b1 to b5 are obtained. As a result, one incident laser beam B is converted into five outgoing laser beams b1 to b5 by the beam splitter 14.
Meanwhile, it is expected for practical application that the thermal stress scribing of the glass plate will be superior in manufacturing processing speed and processed quality in comparison with the conventional mechanical scribing method. The laser scribing method is far superior to the conventional mechanical scribing method in processed quality. In speeding up of the manufacturing processing speed, it is more important to speed up the scribing speed process itself than to shorten management times between each manufacturing process. When only the laser scanning speed is increased while keeping other conditions constant, the thermal stress generated by the laser beam is weakened because of dropped small lower irradiated energy thereof per unit time, so that the power for scribing the glass plate is also weakened. However, we supposed that even if the laser power is small, the laser energy power may not be lowered by controlling the laser beam spot to be very small, so that local heating temperature by the laser beam spot may not be lowered. Then, we tried to scribe the glass plate by scanning a very small laser beam spot at high speed. We came to a conclusion from the trial that it is impossible to increase the scribing speed for scribing the glass plate even if the local heating temperature is heightened by controlling the laser beam spot size to be very small.
We identified that the thermal stress generated by the laser beam depends on not the energy density but total energy added to the heating area.
Then we tried to increase both of the laser beam power and the laser beam scanning speed at the same rate, keeping the cross-section area of the laser beam constant. In this case, the laser beam energy incident to a unit area of the glass is maintained constant and therefore both of the thermal stress and breaking power are not decreased when increasing scanning speed. By this method, it is possible to increase the scribing speed. However, we cannot employ this method because the heating temperature of the glass is so increased that the glass is nondurable when applied to the flat panel display.